1. Field of the Invention
The present invention relates to an active matrix substrate for use in a display device, for example, and also relates to a display device including an active matrix according to the present invention and a method for driving such a display device.
2. Description of the Related Art
A conventional active-matrix-addressed liquid crystal display device has a liquid crystal panel structure including an active matrix substrate and a counter substrate. A counter electrode, commonly used for a plurality of pixels, is formed on the counter substrate. The pixels, data lines for supplying display signals to the pixels, switching elements associated with the respective pixels, and gate lines for controlling the operations of those switching elements are formed on the active matrix substrate. A structure like this will be herein referred to as a “first conventional structure” for the sake of convenience. In this first conventional structure, external drivers such as source and gate driver ICs (which will herein sometimes be referred to as “drivers” collectively), each having the same number of output terminals as that of the data or gate lines, are provided for the liquid crystal panel to drive the data and gate lines.
However, an alternative display device structure was proposed to reduce the number of external drivers needed and the cost of mounting those drivers on the liquid crystal panel. In that alternative structure (which will be herein referred to as a “second conventional structure” for convenience sake), the number of integrated circuits (ICs) is cut down to a half or a third, each output of each of the ICs is branched into two or three, and an appropriate one of those outputs is selectively supplied via a data line switching element. Hereinafter, this structure will be described in further detail.
As in the normal liquid crystal panel (i.e., the first conventional structure), the far ends of the data lines are also electrically connected to their associated source driver ICs, which are among the external drivers needed for the second conventional structure. In this second structure, however, the number of source driver ICs needed is halved, for example. In addition, a data line branching section is also connected to those source driver ICs so as to branch each output of the source driver ICs into two and thereby match the number of branched outputs of the source driver ICs to that of the data lines. And data line switching elements are provided between the data line branching section and the ends of the data lines. Furthermore, a control signal input line is connected in common to the gates of its associated ones of the data line switching elements that belong to a single block. A control signal, which selectively turns ON or OFF the switching elements, is supplied through the control signal input line. The outputs (signals) of each source driver IC, which have been branched by the data line branching section, are supplied to their associated data lines on a sequential basis.
This second conventional structure is disclosed in Japanese Laid-Open Publication No. 8-234237, for example. According to this publication, this structure is advantageous not only in that the number of external drivers required can be reduced but also in that the external drivers can be easily mounted along a single side. In the technique described in this publication, one of the gate lines is selected on a block-by-block basis, while one of two adjacent data lines is selected using a switching element. However, even if one of the data lines is selected on a block-by-block basis, the same effects and advantages are achievable and each of the external drivers can have its configuration rather simplified.
A third conventional structure for an activematrix-addressed liquid crystal display device was proposed in U.S. Pat. No. 4,694,287, for example. The display device disclosed in this patent includes: a counter substrate, on which not the common counter electrode but the data lines are provided; and another substrate on which pixel electrodes, common lines for applying a predetermined potential to pixels, switching elements for the respective pixels, and gate lines for driving the pixels are formed. According to the patent identified above, when the data lines and the gate lines are formed on mutually different substrates, a decreased capacitance is associated with each of those data lines to reduce the load on the driver and a short-circuit failure, usually occurring at an intersection between the data and gate lines, is avoidable.
However, in the first conventional structure in which the pixel electrodes, data lines, switching elements and gate lines are all formed on the same substrate, when one of the pixels is selected responsive to a scan signal supplied through one of the gate lines, the scan signal changes from logical one state to logical zero state. At this particular point in time, the potential level of the liquid crystal layer is slightly dropped by a parasitic capacitance formed between the gate line and the pixel electrode. To avoid image persistence (i.e., undesired image retention), a voltage to be applied to the liquid crystal layer should have its DC component removed. Accordingly, in the first structure, the potential level of the counter electrode on the counter substrate should be regulated with that drop in potential taken into account. The slight drop in potential is also observed in the third conventional structure when a gate line changes from a selected state into a non-selected state. In that case, the potential to be applied to the common lines or the potential levels of the data lines on the counter substrate should be regulated.
Furthermore, the second conventional structure, in which the pixel electrodes, data lines, pixel switching elements and gate lines are formed on the same substrate and in which a video signal (or data signal) is supplied to one of the data lines via a data line switching element, also has the following problems. Specifically, not only 1) when the scan signal changes from logical one into logical zero but also 2) when the data line switching element changes from a selected state into a non-selected state, a parasitic capacitance, which has been created between the control signal input line for the switching element and the data line, drops the potential level of the liquid crystal layer. That is to say, this drop in potential level is superposed on the voltage to be applied to the liquid crystal layer. In addition, the pixel electrodes and the data lines are formed on the same substrate through the same fabrication process. Accordingly, the data line switching elements are of the same type as the pixel switching elements. Thus, the data line and pixel switching elements are both selected (i.e., turned ON) when the scan signal is logical one, and are both non-selected (i.e., turned OFF) when the scan signal is logical zero. Therefore, the drop in potential in the situation 1) and the drop in potential in the situation 2) have the same polarity. For that reason, a voltage, which is greatly different from the direct current component of the signal on the data line (i.e., a display signal), should be applied to the counter electrode to compensate for the drop in potential level.
Furthermore, to allow the driver to decrease its breakdown voltage or to further reduce the power dissipation thereof, a voltage having a waveform of constant amplitude (which will be herein referred to as a “correction voltage”) is usually applied to the counter electrode (or common electrode) in a situation where the data line driver (i.e., the source driver IC) should have a further narrowed drive voltage range. As for the third conventional structure, however, the correction voltage is applied to the common lines, not to the counter electrode.
In this case, a voltage to be applied to the data lines is relatively determined with respect to the correction voltage and changed with the type of the image to be displayed. For example, suppose variations in voltages to be applied to the counter electrode and the data lines with time are observed in a situation where a still image (or fixed image) is displayed. In this case, a voltage of constant amplitude (i.e., the correction voltage) is applied to the counter electrode, while a voltage, having a waveform of which the phase and amplitude have been so controlled as to be of common or opposite to those of the correction voltage depending on the type of the image to be displayed, is applied to the data lines. If the correction voltage has a relatively great level, then the voltage to be applied to the data lines can fall within the amplitude range of the correction voltage in any display mode. However, if the potential level of the liquid crystal layer is dropped significantly by both the pixel and data line switching elements, for example, the drive voltage range of the data lines may sometimes exceed that of the counter electrode. In that case, the voltage should be generated by newly providing another power supply for the data lines separately from the power supply for the counter electrode, thus increasing the power dissipation disadvantageously.
When the potential level of the liquid crystal layer is dropped by a switching element, it depends on the magnitude of a parasitic capacitance unique to the switching element how much the potential level is dropped. However, in a normal manufacturing process of liquid crystal panels, the magnitude of a parasitic capacitance of switching elements is often changeable from one cell to another. This is because in a normal manufacturing process, the gate insulating film of transistors functioning as the switching elements belonging to one cell often has a thickness or line width different from that of the gate insulating film of transistors belonging to another cell. The thickness or line width often shifts from one lot to another during the manufacturing process or from one substrate to another within the same lot. The shift is also caused by a positional displacement of a cell being placed on the substrate.
When a direct current (DC) component is applied to the liquid crystal layer, the reliability might decrease (e.g., the image persistence might occur). Accordingly, to eliminate the DC component, highly precise adjustment is required by using a counter voltage (or correction voltage) regulating volume that is provided externally for each display device.
However, in the second conventional structure in which the dedicated switching elements are provided for the data lines, the potential drop caused by the pixel switching elements is superposed on the potential drop caused by the data line switching elements as described above. Accordingly, the voltage to be applied to the liquid crystal layer is changeable more greatly. As a result, the counter voltage level (or the correction voltage level) should be regulable in a broader range. For example, two volumes may have to be provided: one for regulating the counter voltage level roughly and the other for regulating the voltage level finely. In that unwanted situation, the number of parts required increases and the cost also increases disadvantageously. Furthermore, if the potential drop is so great that the correction voltage level should have a range covering both positive and negative domains across the ground level, then two regulators for positive and negative potentials should be prepared, thus further increasing the manufacturing cost adversely.